1. Field of the Invention
The present invention relates to apparatus employing plural disk units, and particularly to a technology which is effectively applied to a mounting process or the like in a plural disk unit apparatus that includes a plurality of disk units controlled through a bus connection.
2. Description of the Related Art
In rotating external memories such as magnetic disk apparatus used for information processing systems, there is a continuing need for increased storage capacity and reliability, yet at a reduced cost. To meet such demands, a disk array (for example, as described in Pattersonc et al, "A Case for Redundant Arrays of Inexpensive Disks (RAID)", Report No. UCB/CSD 87/391, Computer Science Div., University of California, Berkeley, 1987) having a large capacity and highly reliable external memory has been devised, which is composed of a plurality of small rotating memories which are effectively integrated.
This plural disk unit has a limited disk unit-mounting density due to a load interval (interval between adjacent disk units connected to a small computer system integration (SCSI, or "scuzzy") control bus) problem. More concretely, the SCSI bus most commonly used for a plural disk unit must be designed so that the tap-off length from the bus line is shortened, to ensure a high-speed control bus signal; however, the tap-off length must be somewhat long so that the disk unit is connected to the SCSI bus by a single connector, but when the tap-off length is excessively long, data loss occurs because the pulse of a signal switched to high speed is disturbed by a reflected wave. In particular, the relationship between load interval and tap-off length is important; that is, the load interval must be sufficiently larger than the tap-off length.
There are at least two methods by which to satisfy the relationship between the load interval and tap-off length in high density mounting. One method shortens the tap-off length by providing two control bus connectors on each disk unit and connecting the individual disk units substantially in series to the control bus, and the other increases the load interval by connecting adjacent disk units one after another by means of a flat cable. The former method, however, is disadvantageous in that the provision of two connectors for each disk unit obstructs the high density mounting of the disk units and increases the cost. On the other hand, the latter method is disadvantageous in that connection between disk units one after another does not allow the connectors to be inserted and removed easily; hence, the disk units cannot by easily exchanged by plugging, thus reducing the maintainability of the apparatus.
The entire plural disk unit cannot be adequately miniaturized solely by integrating the disk units; miniaturization must include a general integration of disk units and peripheral units such as power supplies, cooling units, and the like. Furthermore, any attempt at miniaturization requires an effective plan to cool the plural disk unit because heat generation is concentrated for high-density disk units.
Another consideration in the design of a plural disk unit apparatus is the need for uninterruptible operation and high processing speed. To this end, attempts have been made to share a plural disk unit among a plurality of host computers, and to provide uninterruptible operation and uninterruptible maintenance of the plural disk unit itself.